1. Field of the Invention
The present invention relates to a method for operating an internal combustion engine equipped with a piston pump as a high-pressure pump, which is driven by a drive shaft of the engine; the high-pressure pump delivers fuel from a low-pressure region to a high-pressure side and a quantity control valve sets the quantity of fuel delivered by the high-pressure pump.
2. Related Art
In direct-injection gasoline engines (GDI=gasoline direction injection), one-cylinder high-pressure pumps are used to raise the pressure from the preliminary pressure of the presupply pump (EFP=electric fuel pump) to the pressure required for the direct injection (50 to 200 bar). These one-cylinder pumps are operated with 2, 3, or 4 pump strokes per camshaft rotation, depending on the amount of fuel that the motor requires. Usually, the driving action is provided by a cam on the camshaft. During normal operation, each pump stroke is used and the required quantity is set, for example, by a quantity control valve. In other words, when operating in idle mode and in the partial load range, only part of the possible quantity per pump stroke is delivered.
EP-1327766-A2 has disclosed a method in which only a part of the delivery strokes is used at low supply quantities. The motivation for this is the better controllability at very low supply quantities. In this method, a fixed pattern of used and unused delivery strokes in relation to the camshaft rotation is set, e.g. only 2 out of 4 delivery strokes are used.
3. Problems of the Prior Art
When in delivery mode, the high-pressure pump generates structure-borne acoustic vibrations, which generate airborne sound that is perceived as acoustic noise. The method is intended to reduce the acoustic emission of the high-pressure pump and to change this acoustic emission so that it is not perceived as annoying.